The overall objective of this proposal is to determine the regulatory influence of lipopolysaccharide (LPS) from B. fragilis on the lymphoreticular system. For these studies we will compare the biologic effects of LPS from Bacteroides fragilis to those of "classical" E. coli LPS. Bacteroides LPS (B-LPS) was chosen because of its unique chemical nature and relevance to the host. Bacteroides species are one of the predominant anaerobic microorganisms found in the intestinal tract. B-LPS differs enough chemically from E. coli LPS to offer a unique model to study the mechanism by which LPS affects the immune response. The models employed in this study will include 1) germfree versus conventional mice, 2) LPS responsive versus LPS nonresponsive mice, and 3) nude mice versus conventional mice. Since the gut-associated lymphoreticular tissue (GALT) is continually sensitized by microbial products, it is postulated that B-LPS will have a marked effect on Peyer's patch cells and the IgA immune response. Recent evidence from this laboratory has demonstrated that germfree and LPS nonresponsive mice exhibit elevated IgA immune responses following oral administration of antigen. This high IgA response can be abrogated in germfree mice by prior intubation with "classical" LPS or by establishing a gram negative flora (e.g., E. coli) in their intestinal tract. The high IgA responses observed in LPS nonresponsive mice has been shown to be genetically linked to the inability to respond to LPS. Preliminary studies indicate that germfree and LPS nonresponsive mice lack a regulatory T cell population which will suppress the response to orally administered antigen. These studies will investigate the effects of B-LPS on the maturation of the regulatory T cells responsible for oral tolerance. This will be assessed by examining oral tolerance induction, mitogenesis, adjuvancy and the toxicity of B-LPS in the above mouse models. It will be determined whether B-LPS affects T cells, B cells, macrophages or all three cellular types. It is believed that the proposed studies offer a unique approach to understanding 1) the mode of action of LPS and 2) the mechanism regulation oral tolerance.